Serotonin N-acetyltransferase



Most organisms experience a 24-hr cycle due to the environmental dark/light cycle. Melatonin is one of the output signals which causes these rhythms. Circulating melatonin is higher at night acting as a downstream as well as a feedback signal for the biological clock. Melatonin is produced in the pineal gland by two enzymes: 1) serotonin N-acetyltransferase or arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT). The AANAT catalyzes the transfer of an acetyl group from acetyl coenzyme A (AcCoA) to the primary amine of serotonin, producing the product, N-acetylserotonin, which is then methylated by HIOMT to produce melatonin.  The determination of the structure of serotonin N-acetyltransferase is important in order to understand the nature of substrate binding and to know the mechanism of catalysis.  Further,  it helps in designing compounds that inhibit catalysis and prevent proteolysis for treatment of diseases which are melatonin-related (e.g. sleep disorder and jet lag) and serotonin-related (e.g.depression and obesity).

Structure
The structure of the uncomplexed form of serotonin N-acetyltransferase consists of eight beta strands that form a highly twisted beta-sheet and flanked by five alpha-helices. The beta strands are all anti-parallel except for strands beta 5 and beta 6. It can be viewed as two sheets sharing two central strands. A long alpha helix is present at the concave surface (PDBID:1b6b). The structure of serotonin N-acetyltransferase when bound to the bisubstrate analog consists of seven beta strands and five alpha helices. Upon substrate binding, there is a large conformational change in the enzyme; a small beta strand (strand beta 2) is gone, and an alpha helix is extended in that region. The bisubstrate analog is coenzyme A-S-acetyltryptamine. It has the same structure as serotonin except for a hydroxy group at the 5-position of the indole ring. The size and shape of the analog fit into the protein's cavity and the analog is held in an "S" shaped conformation. It is stabilized by three hydrogen bonds : Tyr-168, Leu-124 , and Met-159. The indole ring of the tryptamine group is stabilized by hydrophobic interactions with Phe-56, Pro-64 , Met-159 , Val-183 , Leu-186 , and <scene name='Serotonin_N-acetyltransferase/Hydrophobic/5'>Phe-188 forming a <scene name='Serotonin_N-acetyltransferase/Hydrophobic/7'>hydrophobic pocket for serotonin-binding site. <scene name='Serotonin_N-acetyltransferase/Activesite/1'>His-120 and <scene name='Serotonin_N-acetyltransferase/Activesite/2'>His-122 are conserved residues in the active site.

Implications for Catalysis
Water molecules lying between the tryptamine moiety and histidines facilitate nucleophilic attack on Acetyl Coenzyme-A (AcCoA). It can either serve as a sink for a proton or as a pathway to carry the charge away from the substrates.